Landing at 3, best case you get out of the airport around 3:30.
I'd go a pick up snorkel gear (assuming you have your own) and we were happy with Snorkel Bob. If you stay in Waikiki, then just find somewhere there to grab a drink and watch the sunset. IIRC, think the Hale Koa Bar sells to non-military and is cheap.
Early the next morning go down to Hanauma Bay when it opens (6:30ish) for an hour or two.
Then head to Diamond Head on your way back towards downtown. Plan at least an hour to an hour and a half to hike.
Next head to Punch Bowl Cemetery in place of the Arizona Memorial. Punch Bowl is Hawaii's equivalent of Arlington National Cemetery and includes a few monuments with a great view of Honolulu and the ocean. This way you can support the troops without worrying about tickets, parking, etc.
After that it will probably be close to time to head to the airport to return the car, etc.